The traditional focus of management in the Great Barrier Reef (GBR) catchments has been on reducing the loss of sediment and dissolved inorganic forms of nitrogen (DIN). The impact of nutrients on the Reef is dependent on their bioavailability, so as DIN is immediately bioavailable it was the logical fraction of total nitrogen (TN) to focus on. Whilst DIN is important due to its immediate bioavailability, monitoring of end of catchment nutrient loads has shown that particulate and dissolved organic nutrients comprise the majority of the end of catchment nutrient load (Garzon-Garcia et al. 2015, Turner et al. 2013, Turner et al. 2012, Wallace et al. 2015, Wallace et al. 2014, Waters et al. 2016). Recent research, including Phase 1 of this project, indicates that a portion of the particulate nutrients will be transformed to DIN in short timeframes (1-7 days were chosen as benchmarks) both in fresh and marine waters, thereby becoming bioavailable to aquatic organisms such as algae (Burton et al. 2015, Garzon-Garcia 2014). It was also found that measurements of particulate nitrogen (PN) and particulate phosphorus (PP) are not good indicators of potentially bioavailable nitrogen (N) and phosphorus (P) pools (Burton et al., 2015). As a result, Phase 2 of the Sources of Bioavailable Particulate Nutrients project (RP128G) was developed. The key objectives of Phase 2 were to:

• determine which are the best indicators of particulate nutrient bioavailability to algal and diatom growth in fresh and marine water over a short timeframe
• develop a new, rapid bioassay for testing the effect of particulate nutrients on algal growth over a short timeframe
• provide preliminary information on whether there is an algal response to N and/or P along a freshwater to marine gradient.